1. Field of the Invention
This invention relates generally to storage and dispensing systems for the selective dispensing of fluids from a vessel in which the fluid component(s) are sorptively retained by a solid sorbent medium, and are desorptively released from the sorbent medium in the dispensing operation. More particularly, the present invention relates to a system for manufacturing storage and dispensing apparatus of such type, in which the sorbent is fully prepared and loaded with sorbate prior to its introduction to the storage and dispensing vessel.
2. Description of the Related Art
In a wide variety of industrial processes and applications, there is a need for a reliable source of process fluid(s) which is compact, portable, and available to supply the fluid(s) on demand. Such processes and applications include semiconductor manufacturing, ion implantation, manufacture of flat panel displays, medical treatment, water treatment, emergency breathing equipment, welding operations, space-based applications involving delivery of liquids and gases, etc.
U.S. Pat. No. 4,744,221 issued May 17, 1988 to Karl O. Knollmueller discloses a method of storing and subsequently delivering arsine, by contacting arsine at a temperature of from about -30.degree. C. to about +30.degree. C. with a zeolite of pore size in the range of from about 5 to about 15 Angstroms to adsorb arsine on the zeolite, and then dispensing the arsine by heating the zeolite to an elevated temperature of up to about 175.degree. C. for sufficient time to release the arsine from the zeolite material.
The method disclosed in the Knollmueller patent is disadvantageous in that it requires the provision of heating means for the zeolite material, which must be constructed and arranged to heat the zeolite to sufficient temperature to desorb the previously sorbed arsine from the zeolite in the desired quantity.
The use of a heating jacket or other means exterior to the vessel holding the arsine-bearing zeolite is problematic in that the vessel typically has a significant heat capacity, and therefore introduces a significant lag time to the dispensing operation. Further, heating of arsine causes it to decompose, resulting in the formation of hydrogen gas, which introduces an explosive hazard into the process system. Additionally, such thermally-mediated decomposition of arsine effects substantial increase in gas pressure in the process system, which may be extremely disadvantageous from the standpoint of system life and operating efficiency.
The provision of interiorly disposed heating coil or other heating elements in the zeolite bed itself is problematic since it is difficult with such means to uniformly heat the zeolite bed to achieve the desired uniformity of arsine gas release.
The use of heated carrier gas streams passed through the bed of zeolite in its containment vessel may overcome the foregoing deficiencies, but the temperatures necessary to achieve the heated carrier gas desorption of arsine may be undesirably high or otherwise unsuitable for the end use of the arsine gas, so that cooling or other treatment is required to condition the dispensed gas for ultimate use.
U.S. Pat. No. 5,518,528 issued May 21, 1996 in the names of Glenn M. Tom and James V. McManus, describes a gas storage and dispensing system, for the storage and dispensing of gases, e.g., hydride gases, halide gases, organometallic Group V compounds, etc. which overcomes various disadvantages of the gas supply process disclosed in the Knollmueller patent.
The gas storage and dispensing system of the Tom et al. patent comprises an adsorption-desorption apparatus, for storage and dispensing of gases, including a storage and dispensing vessel holding a solid-phase physical sorbent, and arranged for selectively flowing gas into and out of the vessel. A sorbate gas is physically adsorbed on the sorbent. A dispensing assembly is coupled in gas flow communication with the storage and dispensing vessel, and provides, exteriorly of the vessel, a pressure below the vessel's interior pressure, to effect desorption of sorbate from the solid-phase physical sorbent medium, and flow of desorbed gas through the dispensing assembly. Heating means may be employed to augment the desorption process, but as mentioned above, heating entails various disadvantages for the sorption/desorption system, and it therefore is preferred to operate the Tom et al. system with the desorption being carried out at least partially by pressure differential-mediated release of the sorbate gas from the sorbent medium.
The storage and dispensing vessel of the Tom et al. patent embodies a substantial advance in the art, relative to the prior art use of high pressure gas cylinders. Conventional high pressure gas cylinders are susceptible to leakage from damaged or malfunctioning regulator assemblies, as well as to rupture and unwanted bulk release of gas from the cylinder if the internal gas pressure in the cylinder exceeds permissible limits. Such overpressure may for example derive from internal decomposition of the gas leading to rapid increasing interior gas pressure in the cylinder.
The gas storage and dispensing vessel of the Tom et al. patent thus reduces the pressure of stored sorbate gases by reversibly adsorbing them onto a carrier sorbent, e.g., a zeolite or activated carbon material.
In the manufacture of storage and dispensing systems of the foregoing type, the preparation of the vessel involves degassing same, by high temperature bake-out. Correspondingly, the sorbent medium itself is desirably subjected to elevated temperature conditions prior to contacting the sorbent with the sorbable fluid, so that water, atmospheric gases, other previously sorbed gases, etc., are removed from the sorbent pores and surface area, to maximize the subsequent loading of the sorbable fluid on the sorbent medium.
Accordingly, it has been common practice in the art to load the sorbent material in the cylinder or other vessel to be employed in the storage and dispensing system, and to carry out a high temperature bake-out of the vessel, so that both the vessel and the sorbent therein are simultaneously degassed, in preparation for the subsequent introduction of sorbable fluid into the vessel for contacting with and sorption on the sorbent material.
While economical in terms of carrying out both heating steps at once, the heating of the cylinder and the sorbent therein requires significant cool-down time for the system to reach a temperature at which the contacting of the sorbent with the sorbable fluid can take place. This is due to the fact that the equilibrium capacity (loading) of the sorbent for the sorbate is a decreasing function of temperature, so that the higher temperatures of the bake-out step are highly unfavorable to high loading of sorbate on the sorbent.
In this respect, the vessel employed in the storage and dispensing system is typically a metal walled cylinder, and the thermal mass of the cylinder is therefore considerable. Although the cylinder by itself could easily be chilled to ambient at relatively high rates of cooling, the fact that the sorbent bed in the cylinder contains significant interstitial void space between the packed particles of sorbent in the bed means that the heat transfer from the hot sorbent to the ambient surroundings will be slow.
A further difficulty in the manufacture of storage and dispensing systems of the foregoing type is that the loading of the sorbable fluid on the sorbent is exothermic in character, and therefore the bed will increase in temperature. For a sorbable fluid such as arsine, the temperature rise in contacting a bed of sorbent with the gas may be on the order of 100 degrees Centigrade or even more. Under these circumstances, the fill operation for the sorbent exposure to the sorbable fluid must be carried out slowly or in stages, and/or substantial time is required to cool out the sorbent to ambient temperature. The retardation of the sorbate loading process to accommodate such heat of sorption effects, or alternatively the slow cool-down to ambient temperature occurring when the sorbate is loaded all at once, are substantial impediments to the high speed, large volume manufacturing of the storage and dispensing system units.
It would therefore be a significant advance in the art, and accordingly is an object of the invention, to provide a means and method for manufacturing storage and dispensing systems of the foregoing type, which avoids the need for long-term cool-down of the vessel and sorbent subsequent to degassing thereof, and which substantially eliminates the adverse heat of sorption effects which have hindered prior art efforts to achieve high volume manufacturing of the storage and dispensing systems described hereinabove.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure.